Differential transformer



y 9, 1950 r w. D. MACGEORGE 2,507,344

- DIFFERENTIAL TRANSFORMER Filed Jan. 9, 1948 DISPLACEMENT VOl-TS ,"w'g. 6'.

DISPLACEMf/VT DISPLACEMENT 3nnentor W/ LLIAM D. MACGE ORG E Cfttomcg Patented May 9, 1950 2,507,344 DIFFERENTIAL TRANSFORMER William D.- Macgeorge, Havel-town, Pa., assignor to Automatic Temp Philadelphia, Pa., vania crature Control 00., Inc., a corporation of Pennsyl- -Application January 9, 1948, Serial No. 1,347

1 Claim.

This invention relates to differential transformers, and particularly to means and methods of enhancing the linearity of response and to adjust the zero or null point thereof.

Movable core differential transformers, as typifled, for instance, by the transformer disclosed in Patent No. 2,427,866, have as a stator a pair of co-axial secondary coils coupled in bucking series, symmetrically disposed in axially spaced relation on opposite ends of and relative to a common exciting primary. As the relatively movable armature passes from asymmetry with the primary'in one sense through symmetry to asymmetry with the primary in the opposite sense, while the primary is energized by a suitable A. C. source, the output of the secondaries varies from a voltage of A phase, through a null output, to a voltage of 13 phase. The output of such transformers is generally considered to be substantially linear, and for many purposes is sufficiently close to linear as to be adequate. However, it is found that the actual curve of the voltage output varies as much as perhaps one percent more or less from the truly linear.

It will be understood further of such transformers that the range of relative axial motion of the armature thereof is very minute indeed in securing the desired change in output as a measurement, for instance, of such microscopic motion. With such minute armature motion (which may be of the order of .00002") it will be seen that the adjustments of the stator coils relative to the armature or of the armature relative to the actuating device for moving the armature in order to accurately position the armature at the exact symmetrical point in the stator at which null output is attained, as a starting point for the measurement, will be very delicateindeed. In fact, so delicate that accurate mechanical adjustments are all but impossible, or so complicated as to negative or restrict their use.

It is among the objects of this invention to improve the performance characteristics of differential transformers; to provide means and methods for enhancing the linearity of performance of differential transformers; to provide means for electromagnetically adjusting the coils and the armature of movable armature differential transformers; to provide adjuncts for differential transformers; to obviate or minimize some of the objections to prior art differential transformers; and other objects and advantages will become more apparent as the description proceeds.

In the accompanying drawings forming part of this description:

Fig. 1 represents more or less schematically a longitudinal vertical section through the differ ential transformer according to a preferred but illustrative form thereof, showing a slug or disc or magnetizable material adjustably positioned on one end thereof to enhance the linearity of the output thereof;

Fig. 2 represents an illustrative side elevation of the adjustable slug or disc of the invention;

Fig. 3 represents an illustrative highly exaggerated curve of the output of a typical illustrative differential transformer such, for instance, as is shown in Fig. 1, prior to or without the addition of the metal slug or disc of the invention;

Fig. e represents a similar exaggerated curve of the same transformer after the insertion of the metal slug of the invention as shown in Fig. 1;

Fig. 5 represents a similar sectional view of the transformer of Fig. 1, showing two slugs or discs of magnetic material mounted therein respectively at opposite ends thereof to still further flatten the curve of the output thereof;

Fig. 6 represents an illustrative exaggerated curve of the output of the transformer as illustrated in Fig. 5;

Fig. '7 represents a schematic longitudinal section through a still further modified form of the transformer, showing an axially adjustable magnetizable metal element foreffecting substantially Vernier adjustments of the armature of the transformer in the establishment of a zero or null output thereof, without changing the adjustment of the armature relative to its causative moving device, or the adjustment of the stator relative to the armature;

Fig. 8 represents an end elevation of the insert device of Fig. 7.

The illustrative transformer of Figs. 1, 5 and '7 comprises preferably, although clearly not essentially, an insulating central supporting form, illustratively of a ceramic or like nature, for instance, comprising a cylinder Ill having a plurality of radial annular fins Ii. The fins form partitions to separate a central primary coil l2 from the concentric secondary coils l3 and i l. The latter are coupled in series opposition or bucking relation. The primary coil [2 is arranged for coupling to a source of A. C. A movable core or armature I5 is coupled by a link [6 to a suitable source of relatively slight movement (not shown), such, illustratively as shown in said patent, or as'shown in Hornfeck Patent No. 2,420,539, or in any of applicant's co-pending applications. Obviously, any desired actuating device may be associated with the link l6 as the moving element,

the motion which is to be measured or signalled. It is desired that the motion oi the link it, and thus of the armature it be utilized to develop a potential or a null in the output of the secondaries, which is a function or the motion or the armature or core. Preferably, the ends of the insulating cylinder iii are internally threaded, as at It and respectively.

In the curve of Fig. 3 in a highly exaggerated way, the theoretical linear response of the transformer of Fig. i, as so far described, is indicated by the dot and dash line, and this is supposed to represent the change of voltage as a function of ture displacement. 'Ihe departure from the truly linear in the actual output voltage of the transformer is indicated by the exaggerated solid line curve above it. It is exaggerated to indicate, by the wide deviation shown irom the linear, an actual deviation of approximately 1%, which, if drawn to actual scale might be unnoticeable. he usually but. one end of the curve is used, the deviation from linearity from the start of the curve to the substantial center thereof, starting at the left end of the curve, for instance, is appreciable,

The deviation from the truly linear, that is the deviation from linear of the ratio of the change in induced voltage to the change in armature position, is probably due to the fact that the advancing end of the armature moving away from the null symmetrical position toward an asymmetrical position of a potential output from the bucking secondaries is moving away from the most intense field or magnetic flux. This inven= tion contemplates the provision of an additional path of flux with which the advancing end of the armature has increasing response even though leaving or advancing beyond th most intense field centered in the primary coil and in the adjacent mass or the armature. Such deviation being the ratio heretofore recited.

To secure the additional flux path in an em cient and preciseway, a slug or disc 20 is provided, of magnetic material having th external threads 22, and also having internally the substantially diametrical slot or other recess opening at one edge in order to reduce eddy currents. The slug 2i is threaded into the threads is at the end or the cylinder it of the transformer, adjacent to the free end of the armature, and lies in a plane diametricai of the cylinder, parallel to the transverse extent of the armature. The magnetic disc or slug forms an auxiliary or supplemental path for magnetic flux developed in the armature from the primary coil, and forms with the armature a. generally closed loop flux circuit so as to maintain an efiective field on the advancing end of the armature. The disc is screwed into the cylinder, usingthe'slot 23 for a tool if desired, until the maximum desired efiect me the ratio or the change in induced voltage to the change in ature position.

If it is desired to have a similar result at the other end oi the curve, a second disc 26, of the same general characteristics as the t, is screwed into the threads as at the right end of the cylinder id, utilizing the slot or recess in the disc or slug as the passage for the motion trans lei-ring link it, as will be clear. This double disc association attach the smoothed and substantially linear path oi voltage output as exaggeratedly indicated in 6, in which both ends of the curve have been affected by the slugs or discs. The improved functioning of the transfoer of this invention for delicate and accurate testing and like responsiveness to small motions, by oftesting a sharper linearity, will be understood.

' centered in the end of the cylinder and axially adjustable to efiectively change the center of the armature to the center of the primary. To this end the rough'rnechanical relative adjustments are so made as to cause the stator to be to the left of center (inll ig. 7). Then a ceramic or other insulating disc is pushed or screwed into the end of the cylinder it, provided with a finethreadecl axial bore 26, in which an elongated threaded shank 2?, preferably in the form of a screw having a slotted or like head 28, is mounted. When an approximation of the symmetrical disposition of the armature 05 in the excitin coil 82 is attained, but, as noted, to the right of such actual center (as the stator is to the left thereof and there is, therefore, a predominant excess of flux in coil id because of the presence of the armature) the plug is inserted, or if previously inserted, the screw is adjusted'inwardly as indi= Fig. '7, eflectively shifts the armature is to the is secured, as a'function of the proximity of the disc or slug to the armature, as it is adjusted inwardly in the cylinder ill. The efiect of the properly positioned slug 2! upon the output of the transformer is indicated in Fig. 4, in a flattening of the actual output curve, with a sharp reduction of the previously existing deviation from the truly linear output. This effective change in output, in contrast to the linear, is indicated in an exaggerated way in said figure. The previous deviation of the illustrative 1% of the voltage from the truly linear is reduced to an illustrative substantially .1% of the voltage. This, as indicated, is most evident at the left end of the curve. The reference to linear deviation left, to vary its efiective center of null output, without actually moving the armature. Of course, if the initial amustment is made with the screw all the way in, and'the adjustment is with the stator to right of center, withdrawal of the screw effects the balance in output of the secondaries without moving the armature. Thus, extremely minute but accurate adjustments of the armature and coils, without disturbing the relation of the armature and the actuating source are accomplished.

Having thus described my invention, I claim:

A diflerential transformer comprising an exciting primary coil and a pair of secondaries 25 of the secondaries to secure a potential, electrovoltage to the change of armature positions, and

a second electro-magnetlc means spaced from the primary on the opposite side thereof from the first mentioned electro-magnetic means and arranged to form a supplemental flux path juxta-' posed to the movable armature to still further 1o 2,427,866

reduce the linear deviation of the ratio or the change of voltage to the change of armature position.

WILLIAM D. MACGEORGE.

6 REFERENCES man The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,340,749 Harvey Feb. 1, 1944 2,364,237 Net! Dec. 5, 1944 2.423.824 Beetham July 15, 1947 Macgeorge Sept. 23, 1947 FOREIGN PATENTS Number Country Date 697,868 Germany June 1, 1934 598,886 Germany June 21, 1934 

